1. Field of the Invention
This invention relates to a magnetic recording medium and more particularly, it is concerned with an improved magnetic tape whose coefficient of friction is lowered without lowering S/N, which has an improved running durability and which is free from edge snick and scraping of the back surface.
2. Description of the Prior Art
Generally, in magnetic recording tapes for audio, video or computer, the surface of the magnetic recording layer is finished smooth so as to improve the sensitivity, in particular, output in high frequency range. However, such a smooth magnetic recording tape has a tendency that it is not taken up regularly during taking-up or rewinding, but it causes often disordered winding. The use of this magnetic tape meets with deterioration of the running property and fluctuation of the output due to tension fluctuation. In addition, this magnetic tape tends to be deformed or damaged. In order to overcome these disadvantages, there has been proposed a magnetic recording tape in which a back layer is provided on the reverse side of a support to a magnetic recording layer. However, the back layer used in the prior art should further be improved, because it often meets with scraping, increase of friction coefficient and folding of tape.
As well known in the art, when a back layer is provided for the purpose of improving the tape running property and running durability, the roughness of the back layer is transferred to the surface of a magnetic layer to degrade the surface property of the magnetic layer, thus deteriorating the electromagnetic property, in particular, S/N property of the magnetic recording medium, in a case where the magnetic recording medium, in particular, in the form of a tape is rolled up or stacked in the form of sheets. That is to say, deterioration of S/N property is due to that the roughness of the back layer is transferred to the surface of the magnetic layer when the magnetic recording medium is maintained or allowed to stand in rolled or stacked state, in which the surface state of the back layer is rough as a result of regarding the running property as important.
In order to overcome the above described disadvantages of the back layer, the inventors have proposed a magnetic recording medium in which a back layer is provided having spike grains with a height of 0.8 to 5.mu. and an interval of at least 200.mu. on the surface thereof and a friction coefficient of at least 0.25.mu. (U.S. Pat. No. 4,135,031). According to this prior art, the problems about the order of tape edges, tape loosening and step-out of edges of a wound tape can considerably be solved, but the problems about S/N property and durability remain unsolved.